(1) Field of the Invention
The present invention relates to a so called xe2x80x9ctandem-typexe2x80x9d image forming apparatus which is provided with a plurality of image forming units and which is capable of operating in a full-color print mode and a single-color print mode. All of the image forming units are operative in the full-color print mode to form a color image, while only one of the image forming units is operative in the single-color mode to form a single-color image.
(2) Related Art
A tandem-type image forming apparatus, as one example of a conventional full-color image forming apparatus, has image forming units corresponding to toner colors set in line. A transport means of a tandem-type image forming apparatus transports a recording sheet to each transfer position (i.e., a position at which a toner image is transferred onto the recording sheet) of the image forming units. Color toner images formed by the image forming units are sequentially superimposed to form a full-color image on the recording sheet.
In general, a tandem-type image forming apparatus has four image forming units corresponding to four colors, that is, cyan (C), magenta (M), yellow (Y), and black (K). The image forming unit used for forming a black image is usually set at a rearmost position of a transportation path of the recording sheet transported by the transport means to prevent blurring of colors. An attraction charger is set at a frontmost position of the transportation path of the recording sheet and charges the recording sheet supplied from a sheet supply unit so that it is attracted to the transport means. The recording sheet is attracted to the transport means by the electrostatic charge of the attraction charger and sequentially transported to each transfer position of the image forming units without moving away from a correct position on the transport means. As a result, the multicolor image can be formed without color displacement.
A transport belt can be used as the transport means. In general, a transport belt is made of a semi-conductive material to encourage the self-elimination of static. The surface potential of the transport belt gradually decreases with time after the transport belt has been charged. For this reason, the electrostatic force which attracts the recording sheet decreases as the recording sheet is transported in a transport direction thereof. However, when a full-color image is formed, the recording sheet and the transport belt are charged with transfer voltage at the transfer positions of the image forming units for cyan, magenta, yellow, and black in addition to being charged by the attraction charger. Consequently, the recording sheet can be reliably transported all of the way to a fixing device, with the attraction being maintained above a predetermined level.
When only one of the image forming units is used, however, such as when forming a black image using only the final image forming unit, the recording sheet may move away from the correct position on the transport means before reaching the transfer position of the image forming unit used for forming the black image since the electrostatic force applied to the transport means decreases due to the self-elimination of static. Alternatively, a paper jam may occur, depending on the type of the recording sheet being used and the conditions inside the image forming apparatus.
Japanese Laid-Open Patent Application No. 4-221982 teaches an example of a tandem-type image forming apparatus which has all of the image forming units apply a transfer voltage even when the image is formed using only some of the image forming units. Thus, the electrostatic force of the recording sheet attracted to the transport means can be sufficiently maintained even when the image is formed using not all of the image forming units, so that the recording sheet does not move away from the correct position on the transport means.
General speaking, however, in organizations such as offices, the number of image formations using only some of the image forming units, such as operations using only the image forming unit for forming a black image, will be considerably higher than the number of image formations using all of the image forming units for forming color images. As in the cited Japanese application, if electrostatic charge is applied the same number of times, regardless of whether the single-color image formation or the full-color image formation, there will be an increase in power consumption in addition to a great acceleration in the wear and tear on the transport means.
Tandem-type image forming apparatuses are usually provided with a variety of modes in addition to the ability to select between the color image formation and the single-color image formation. If electrostatic charge is applied to the recording sheet to be attracted to the transport means the same number of times, regardless of the current mode, there will be an increase in power consumption in addition to a great acceleration in the wear and tear on the transport means.
The tandem-type image forming apparatus operates based on the premise that high-quality paper is used as the recording sheet for better reproduction of color images. By using high-quality paper, the attraction of the recording sheet relative to the transport means is ensured. FIG. 1 is a graph which compares the attraction of high-quality paper, with those of recycled paper and low-grade paper which have rough surfaces.
As shown in the graph, the attraction of recycled paper and low-grade paper are extremely low, compared with the attraction of high-quality paper. As such, when recycled paper or the like is used as the recording sheet in the conventional tandem-type image forming apparatus, the recording sheet is not reliably transported, and so is skewed or moves away from the correct position on the transport means. At worst, the transport means cannot transport the recording sheet, causing a paper jam.
However, there are increasing user demands for the use of recycled paper or other low-grade paper as the recording sheet to reduce costs, since it is not necessary to consider the prerequisites for color reproduction when reproducing a single-color image.
The primary object of the present invention is to provide an image forming apparatus by which the stated problems are solved.
The first object of the present invention is to provide an image forming apparatus which can considerably prevent the short lifespan of the transport unit.
The second object of the present invention is to provide an image forming apparatus which can maintain the transport of the recording sheet stable in each image forming mode by controlling the electrostatic attraction of the recording sheet relative to the transport unit as necessary, and considerably prevent the short lifespan of the transport unit.
The third object of the present invention is to provide an image forming apparatus which can avoid the problem, such as a skew and a movement of the recording sheet on the transport unit, when a sheet whose surface is rough is used as the recording sheet in the single-color image formation.
The above-mentioned objects can be achieved by an image forming apparatus made up of: a plurality of image forming units, each including an image holding component on which an image for a different color is formed and including a transfer component for transferring the image formed on each image holding component onto a recording sheet at a respective transfer position; a transporter for holding a recording sheet and sequentially transporting the recording sheet to each transfer position of the plurality of image forming units; and a controller which sets a system speed of the image forming apparatus at a first speed when a first mode is set to form a color image using the plurality of image forming units and sets the system speed of the image forming apparatus at a second speed which is faster than the first speed when a second mode is set to form a single-color image using only one of the plurality of image forming units, with the system speed being equivalent to a transportation speed of the recording sheet by the transporter.
The above-mentioned objects can be achieved an image forming apparatus made up of: a plurality of image forming units, each including an image holding component on which an image for a different color is formed and including a transfer component for transferring the image formed on each image holding component onto a recording sheet at a respective transfer position; a transporter for holding a recording sheet and sequentially transporting the recording sheet to each transfer position of the plurality of image forming units; a first attraction unit which applies attractive force that attaches the recording sheet to a recording sheet supporting surface of the transporter, the first attraction unit being located at upstream side of an image forming unit which is located at a most upstream side in a transporting direction of the recording sheet by the transporter; a second attraction unit which increases the attractive force of the recording sheet which was applied by the first attraction unit, the second attraction unit being located at downstream side of the first attraction unit in the transporting direction of the recording sheet; and a controller which makes only the first attraction unit operative when a first mode is set to form a color image using the plurality of image forming units and makes both of the first attraction unit and the second attraction unit operative when a second mode is set to form a single-color image using only one of the plurality of image forming units.
The above-mentioned objects can also achieved by an image forming apparatus made up of: a plurality of image forming units, each including an image holding component on which an image for a different color is formed and including a transfer component for transferring the image formed on each image holding component onto a recording sheet at a respective transfer position; a transporter for holding a recording sheet and sequentially transporting the recording sheet to each transfer position of the plurality of image forming units; and a controller which sets a system speed of the image forming apparatus at a first speed when a first mode is set to form a color image using the plurality of image forming units, and sets the system speed of the image forming apparatus at a second speed which is slower than the first speed when a second mode is set to form a single-color image using only one of the plurality of image forming units and special paper aside from plain paper is used as the recording sheet, with the system speed being equivalent to a transportation speed of the recording sheet by the transporter.
The above-mentioned objects can be achieved by an image forming apparatus made up of: a plurality of image forming units, each including an image holding component on which an image for a different color is formed and including a transfer component for transferring the image formed on each image holding component onto a recording sheet at a respective transfer position; a transporter for holding a recording sheet and sequentially transporting the recording sheet to each transfer position of the plurality of image forming units; and a controller which sets a transfer pressure at a first pressure for each of the plurality of image forming units when a first mode is set to form a color image using the plurality of image forming units, and sets the transfer pressure at a second pressure which is higher than the first pressure for the image forming units which are not used for forming a single-color image when a second mode is set to form the single-color image using only one of the plurality of image forming units.
The above-mentioned objects can be also achieved by an image forming apparatus made up of: a plurality of image forming units, each including an image holding component on which an image for a different color is formed and including a transfer component for transferring the image formed on each image holding component onto a recording sheet at a respective transfer position; a transporter for holding a recording sheet and sequentially transporting the recording sheet to each transfer position of the plurality of image forming units; and a controller which controls each transfer component to change an electric field so that an area that is acted upon by the electric field is a first area when a first mode is set to form a color image using the plurality of image forming units, and to change the electric field so that the area that is acted upon by the electric field is a second area which is larger than the first area when a second mode is set to form the single-color image using only one of the plurality of image forming units.